Don't set music.usingmmmmmm to true when (un)loading custom assets

For some reason, music.usingmmmmmm automatically gets set to true in
musicclass::init(). I assume this was because it would get re-assigned
by game.usingmmmmmm in the game startup code anyway, but now that
musicclass::init() can be called more than once, this variable will just
get set to true when it shouldn't be, causing a confusing desync just
like the one I described in my previous commit, where you would have
PPPPPP or MMMMMM on the title screen, but closing the game and
re-launching it would play the other soundtrack instead.

Again, these duplicate variables should be removed, but that's going to
be a separate patch. In the meantime, I'm removing this variable
assignment.

desktop_version/src/Music.cpp

@@ -95,7 +95,6 @@ void musicclass::init()
 	else
 	{
 		mmmmmm = true;
-		usingmmmmmm = true;
 		int index;
 		SDL_RWops *rw;